Mechanistic considerations

HD-reactions are sometimes referred to base-catalysed halogen-dance (BCHD) reactions [1,2], should be considered as aryl-bromide catalysed dance (ABCD) reactions, because the driving force of such rearrangements is the generation of polybromoaromatic intermediates, which serve as transbromination catalysts within a series of metal-exchange reactions, as depicted below (5,5'-dibromo-2,2'-bithiophene 1 used as starting compound).

After initial metalation of 1 by LDA (step [1]) a series of rearrangement steps occur (step [2], [3], [4]) which result in formation of rearranged I3. (Although 5,5'-dibromo-2,2'-bithiophene 1 has two reactive sites, this fact is neglected for simplification in the previous scheme).

From the proposed mechanism it can be easily deferred that formation of rearranged product requires contact of starting compound 1 with the intermediate I1 which can in practise be achieved by

  1. providing excess of starting material
  2. rapid addition of starting material
  3. elevated reaction temperature (approx. -20 C)
  4. reduction of the polarity of solvent (which causes [1] to react more slowly)
  5. inverse addition of base

Usually, application of conditions 1 - 4 enables successful formation of desired I3.

In contrast, opposite measures have to be taken to obtain non-rearranged products.